Influence of cations, sialic acid and pH on the expansion of films of canine lung surfactant.

Sialic acid (14.6 mug/mg protein) was quantitated in the non-cellular material removed from the lung of Beagle dogs by lavage. Sialic acid did not affect the dynamic surface tension properties of either the total alveolar lipid removed by lavage or of its major lipids, dipalmitoyl lecithin (DPL) and dipalmitoyl phosphatidyl glycerol (DPG). The presence of divalent cation (Ca2+, Mg2+, Mn2+ and Zn2+) or a lowered pH in the subphase medium lowered the surface tension during the expansion phase of the total alveolar lipid film when it was compressed and expanded on a Wilhelmy trough. Films of DPL behaved similarly, but no pH effect was observed with DPG monolayers. The cation effect manifested itself in the same direction as the value of the individual stability constants (Zn2+ greater than Mn2+ greater than Mg2+ greater than Ca2+) which suggests ionic binding of the cations to the phosphate group of the phospholipids. A physiological advantage of such an effect may lie in the conservation of the energetically favorable low surface tension state achieved during film compression with a minimum of surfactant lipid.